This invention relates to a medical appliance for use in magnetic resonance imaging procedures performed on a body, comprising an antenna detecting magnetic resonance response signals, the antenna intended to be inserted into the body for interacting with a magnetic resonance procedure for calculating the position of the medical appliance in the body.
Tracking of catheters and other devices positioned within a body may be achieved by means of a magnetic resonance imaging system in order to avoid using X-rays and the risk of accumulated X-ray dose to the patient and long term exposure to the attending medical staff.
Typically, such a magnetic resonance imaging system may be comprised of magnet means, pulsed magnetic field gradient generating means, a transmitter for electromagnetic waves in radio-frequency, a radio-frequency receiver, a processor, and a controller. The device to be tracked has attached to its end a small coil of electrically conductive wire. The patient is placed into the magnet means and the device is inserted into the patient. The magnetic resonance imaging system generates electromagnetic waves in radio-frequency and magnetic field gradient pulses that are transmitted into the patient and that induce a resonant response signal from selected nuclear spins within the patient. This response signal induces current in the coil of electrically conductive wire attached to the device. The coil thus detects the nuclear spins in the vicinity of the coil. The radio-frequency receiver receives this detected response signal and processes it and then stores it with the controller. This is repeated in three orthogonal directions. The gradients cause the frequency of the detected signal to be directly proportional to the position of the radio-frequency coil along each applied gradient.
The position of the radio-frequency coil inside the patient may therefore be calculated by processing the data using Fourier transformations so that a positional picture of the coil is achieved. Since however the coil only reacts, literally not a positional picture of the coil but in fact a positional picture of the position of the response signals inside the patient is achieved. Since this positional picture contains no information yet on the region surrounding the immediate vicinity of the coil, this positional picture can be superposed with a magnetic resonance image of the region of interest. In this case the picture of the region may have been taken and stored at the same occasion as the positional picture or at any earlier occasion.
Radio-frequency antennas in the form of a coil couple inductively to the electromagnetic field and they allow obtaining a substantially spatially uniform magnetic field which results in a relatively uniform image intensity over a wide region. The problem is however that coil configurations are bulky (the received signal is determined by the loop diameter) and cannot be implemented for use in narrow vessels, whereby their use for the placement of medical appliances such as catheters may be critical.
Furthermore, the spot image which is provided for by the coil antenna does not allow knowing or even evaluating the orientation of the device; as a result, the magnetic resonance imaging system cannot be used for steering the device into tortuous areas such as blood vessels.
European Patent No 0165742 describes a catheter for use with magnetic resonance imaging systems. This catheter comprises a sheath which has embedded within the wall thereof a pair of conductors preferably formed of a foil composite obtained by plating of conductive materials of selected magnetic susceptibility to yield a composite of desired susceptibility substantially matching that of the sheath. In this way, the magnetic invisibility of the catheter is maintained. The tip of the catheter contains a loop connecting the conductors, the phase of such a loop being preferably transverse to the catheter symmetry axis. As explained in the document, when excited by a weak pulse source, the loop supports a dipole magnetic field which locally distorts the magnetic resonance image providing an image cursor on the magnetic resonance imaging display, and a low magnetic susceptibility functional element such as a light pipe threaded into the catheter sheath allows direction of the catheter through selected blood vessels. The essence of this structure is thus the accurate location and monitoring of the catheter tip.
However, this is achieved within the environment of a bulky configuration which cannot be advanced through narrow vessels and which cannot be steered by reference to the magnetic resonance imaging system.
The document WO 87/04080 shows surgical catheters composed of alternating annular segments of non-magnetic materials which are highly opaque to nuclear magnetic resonance examination and less opaque, respectively. These catheters have thin coatings of silicone rubber on their external surface as well as on internal surface of their main central lumen. A plurality of further lumens are distributed circumferentially within the catheter wall and guidance wires are housed in said lumens, secured at the distal end of the catheter wall and coupled to a joystick at the proximal end of the catheter for individual tightening and relaxing to permit radial guidance of the distal end of the catheter. The central lumen of the catheter and still further secondary lumens arranged in the catheter wall are for the distribution of various drugs or for surgical tools such as optic fiber for laser surgery or suturing devices or still stitching grippers. By these arrangements, location of the catheters is apparent under nuclear magnetic resonance examination, visually at the distal end. These structures are however bulky and they have the same drawbacks as outlined hereinbefore.
European Patent Application published under N 0385367 shows an insertable prostate pick-up probe devised for being a nuclear magnetic resonance receiving device capable of imaging spectra from the human prostate and surrounding tissue; this probe may also be used as the transmit coil for radio-frequency excitation. This probe is intended to be used with an interface network providing the tuning, impedance matching, and decoupling functions, and including a connection to a magnetic resonance imaging scanner.
The probe includes a shaft supporting a patient interface balloon at its distal end, comprising an inner balloon and an outer balloon, the inner balloon being capable of being inflated with air supplied through a lumen within the shaft. A non-stretchable lane formed of an adhesive backed cloth material partly covers the inner balloon and serves as a guide for a flexible receiving coil arranged between the inner balloon and the outer balloon, this coil being electrically connected to the interface via an insulated cable extending through the shaft. Upon inflation, the non-stretchable plane rises and forces the receiving coil and outer balloon against the region of interest so that the receiving coil is in position to receive the best possible radio-frequency signal from the region of interest. Special indentations forming a shell are provided on the outer balloon to act as coil positioners when the balloon is in its uninflated state so that the coil may be repeatedly positioned relative to the shell inside the outer balloon for numerous clinical inflation and deflation cycles. A colored stripe is marked on the shaft, possibly including a scale, for indicating the distance which the shaft has been inserted into the patient and also the radial orientation of the balloon for proper alignment with the region of interest. In operation, the probe is inserted while the patient interface balloon is in the uninflated state; the alignment stripe marked on the shaft is used to radially and longitudinally position the probe within the region of interest. Once the probe is correctly placed, the patient interface balloon is inflated and the receiving coil is forced against the region of interest. The probe is then connected to the interface network via the insulated cable.
This particular arrangement of the radio-frequency coil does not reduce the bulk of the system which cannot be used for narrow or tortuous vessels. Furthermore, the system does not provide for any information as to orientation of the probe for steering purposes.
The document DE-3937052 A1 shows a biopsy tube for use in a magnetic resonance imaging procedure, comprising longitudinally extending coaxial conductor tubes separated by insulator tubes and extending the length of the biopsy tube. In a further embodiment, the conductor tubes are replaced by gutter like portions of coaxial conductor tubes which are separated by an insulator filling. Here again, the result is a bulky configuration which cannot be advanced to narrow vessels. In addition, that kind of assembly is substantially stiff, thereby further preventing the applicability of the instrument in tortuous vessels.